1. Field of the Invention
This invention relates to optical systems of variable magnification power and more particularly to an optical system of which the focal length is varied by varying the refractive power of at least one lens component.
2. Description of the Prior Art
In prior art variable-magnification optical systems, continuously varying the focal length of the entire system throughout a range while maintaining the position of the image plane constant required axially moving at least two lens components to effect zooming. During zooming these two lens components always have to be separated from each other by a given amount to avoid mechanical interference between them. This necessarily put a restriction on the requirement of minimizing the bulk and size of the lens system. To accomplish the latter end, at least one of the zoom components had to be moved non-linearly. Because of this, a smooth zooming operation was difficult to achieve. Attempts have been made to realize a compact wide-range zoom lens system by using three or more zoom components axially movable in different relations to one another. To hold these components and to make sure these components are smoothly movable, however, the mechanical mounting therefor had to be constructed in a very complicated form. Therefore, the use of three or more zoom components in a single zoom lens system could not be said to be a good solution.
There have been proposals for continuously varying the refractive power (focal length) of a component by varying the radius of curvature of its lens surface or by varying the refractive index of the material from which the component is made, as, for example, disclosed in Japanese Laid-open patent applications Nos. SHO 55-36857, 56-64209, 56-62201, 56,92501, 56-101101 and 56-140303. Also known as an example of application of the refractive power-variable lens to optical systems is Japanese Laid-Open patent application No. SHO 54-146648.